A VH gene that fails to promote development of follicular B cells. We have generated novel lines of mice by introducing the VH genes from 2 anti-arsonate hybridomas into a natural location in the endogenous Igh locus (VH "knockin" mice). 1 of these lines, called HKI65, contains a VH gene that lacks somatic mutations. This VH gene can alone promote normal development of B1, B2 and marginal zone (MZ) B cells, as shown when it is present in the context of an inactivated JH locus on the other allele (JHD). In striking contrast, in another VH knockin/JHD line (termed HKI71) in which the VH gene differs from that in the HKI65 line by only 8 amino acid substitutions, B2 cells are essentially absent, B1 cells are present at near normal levels in the peritoneum, and most B cells in the spleen are of the MZ phenotype. Preliminary analysis of adult bone marrow B cell development in HKI71mice indicates a severe block at or about the pre-B stage of development. Further studies of HKI71/JHD mice promise to provide new insights into the factors that regulate the development of MZ B cells. However, to allow such future studies to be clearly defined, more detailed preliminary data are required. In particular, the nature of the defect in B lymphopoiesis needs to be investigated. In addition, since MZ B cells are thought to have the ability to expand and self-renew in the periphery, whether the predominance of MZ B cells in HKI71/JHD mice results from events secondary to a state of B lymphopenia during the development of HKI71/JHD mice needs to be addressed. In this proposal, 2 focused specific aims are described that will address these issues. [unreadable] [unreadable]